In a dialysis machines and other life support systems a precisely proportioned supply of two liquids is often required to be provided with particular reliability and stability. In a dialyzer for an artificial kidney system, for example, a supply of water is fed in a selected nominal ratio, such as 34:1, relative to a concentrate with which it is mixed. While it is feasible to detect the proportion of the concentrate in the mixture, as by using a conductivity cell, the problems involved in safely and accurately pumping and controlling a variable water-concentrate have not heretofore been satisfactorily overcome.
In most prior art systems the approach has generally been to use either a fixed proportion pumping system or to employ a fully servo controlled system. In the fixed proportion displacement pumping system, the proportion of delivery of each liquid is initially precisely set, but this is not satisfactory for long term operation inasmuch as wear and changes in concentrate characteristics or other factors can cause the pumping system to go out of adjustment. In addition, it is not sufficient merely to deliver a precise ratio of the two liquids on a long term basis, inasmuch as short term factors are also very important. Because piston speeds with common types of proportional drives vary sinusoidally, unequal flows can occur at different times in the stroke, and this cannot satisfactorily be compensated by faster speed operation, which tends to introduce substantial wear and which also introduces surges in the flow. The use of a large mixing chamber is not desirable, because this substantially increases the problem involved in sterilizing the unit.
The servo techniques that have been applied have typically assumed complete authority over the constituent flows, but at the expense of substantially greater risk. The failure of any detector, circuit or component can result in malfunctioning of the equipment and severe harm to the patient. A complete failure of key components is not required, inasmuch as simple malfunctioning of particular components can result in a catastrophic situation from the standpoint of the patient. It is not desirable to rely upon energy sources which are subject to failure, or to utilize pumping systems which do not insure positive displacement of fluid.